Abstract
Organic matter in the sea surface microlayer (SML) may be transferred to the atmosphere as sea spray and hence influence the composition and properties of marine aerosol. Recent work has demonstrated that the SML contains material capable of heterogeneously nucleating ice, but the nature of this material remains largely unknown. Water-soluble organic matter was extracted from SML and underlying seawater from the Arctic and analyzed using a combination of mass spectrometric approaches. High performance liquid chromatography-ion trap mass spectrometry (LC-IT-MS), and Fourier transform ion cyclotron resonance MS (FT-ICR-MS), showed seawater extracts to be compositionally similar across all stations, whereas microlayer extracts had a different and more variable composition. LC-IT-MS demonstrated the enrichment of particular ions in the microlayer. Ice nucleation ability (defined as the median droplet freezing temperature) appeared to be related to the relative abundances of some ions, although the extracts themselves did not retain this property. Molecular formulas were assigned using LC-quadrupole time-of-flight MS (LC-TOF-MS2) and FT-ICR-MS. The ice nucleation tracer ions were associated with elevated biogenic trace gases, and were also observed in atmospheric aerosol collected during the summer, but not early spring suggesting a biogenic source of ice nuclei in the Arctic microlayer.
Highlights
The sea surface microlayer (SML) is a thin layer of water at the sea-air interface in which chemical, physical and biological properties differ from those of the underlying seawater.[1]
As the SML lies at the interface between the ocean and the atmosphere, it is expected to influence the transfer of gases and particles between these compartments.[6−10] Material from the SML may become entrained in sea spray aerosol (SSA) generated by bubble bursting processes,[11] but the extent of the SML’s direct contribution to SSA remains unknown.[12]
Low mass resolution liquid chromatography mass spectrometry was used to explore the molecular composition of dissolved organic matter (DOM) isolated from Arctic SML and underlying seawater, with the aim of identifying features which related to ice nucleation activity, an atmospherically relevant property
Summary
The sea surface microlayer (SML) is a thin layer of water at the sea-air interface in which chemical, physical and biological properties differ from those of the underlying seawater.[1]. Low mass resolution liquid chromatography mass spectrometry was used to explore the molecular composition of dissolved organic matter (DOM) isolated from Arctic SML and underlying seawater, with the aim of identifying features which related to ice nucleation activity, an atmospherically relevant property. Samples were analyzed drying gas (N2) in positive and flow rate negative of 9 L mode separately; in each mode external mass calibration was conducted using Agilent low concentration tuning mix FT-ICR-MS spectra of microlayer samples, seawater samples and procedural blanks within this narrower m/z window were compared to identify ions present at appropriate relative abundances, and possible molecular formulas for these were generated. The reconstitution volume was selected such that analyte concentrations were returned to those in the original, untreated SML sample This allowed direct comparison of the extract IN activity with the microlayer IN activity measured in the raw samples during cruise JR288.29. A suite of trace gases (DMS, halocarbons, monoterpenes) were measured by purge-and-trap gas chromatography mass spectrometry using the method described in,[56] though we caution that these results are semiquantitative at best because microlayer sampling methods were not gastight
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